1. Field of the Invention
The present invention relates to an apparatus for sensing and regulating drop-by-drop flow of liquid. This apparatus has particular value as a medical instrument for automatically sensing and controlling drop-feed of liquids where accuracy is of considerable importance.
2. Description of the Prior Art
In the medical field, drop-by-drop liquid feed systems are commonly used to control introduction of liquids, such as whole blood, liquid plasma, or liquid nutrients, into a patient. A typical intravenous liquid injection system comprises a bottle of liquid supported in an inverted position, an intravenous feed tube connected indirectly to the inverted bottle opening and a valve mechanism such as a manually adjustable clamp that controls the rate at which liquid is allowed to leave the bottle. A transparent drip chamber is ordinarily interposed between the bottle and the feed tube so that a nurse or other attendant can observe the rate at which liquid drops out of the bottle. A pool of liquid is also maintained in the lower portion of the chamber to ensure that no air enters the feed tube or is conducted to the patient.
Various devices for automatically controlling the rate at which liquid drops into the drip chamber have been proposed. For example, U.S. Pat. No. 3,871,229 to John H. Fletcher, inventor of the subject invention, discloses a drop sensing apparatus which comprises a thermally responsive probe sensor mounted inside the drip chamber and electrically insulated from the drop stream. However, the sensor is thermally coupled to the stream to generate pulses at a rate corresponding to the periodicity of the falling drops. The sensed rate is compared with an adjustable preselected rate and any detected difference is utilized to regulate the drop rate. Regulation is accomplished by a motor driven adjustable clamp which variably crimps the feed tube to regulate liquid back pressure in the drip chamber and, in turn, to regulate the drop rate.
This system is high resistant to self-induced failure and to error due to spurious signals yet is highly accurate. Therefore, it is superior to previously devised automatic systems which employ photocell drop sensors (see, for example U.S. Pat. Nos. 3,736,930 (Georgi) and 3,700,904 (Stobbe et al.)), or capacitive drop sensors (see, for example U.S. Pat. Nos. 3,390,577 (Phelps et al.) and 3,545,271 (Amir et al.)) for controlling the rate of liquid flow. Generally, photocell drop sensors are expensive and are subject to inaccuracy which results from extraneous light signals from, for example, a flickering fluorescent light. Moreover, the drip chamber may accummulate deposits of residue on its side walls that obstruct the light beam which actuates the sensor. Capacitive systems exhibit many of the same drawbacks for similar reasons.
However, the advantages provided by the system disclosed in the Fletcher Patent over other prior systems may be even farther improved and enhanced by the present invention.
Motor driven drop rate controls (see, for example, the Fletcher and Stobbe et al. patents), though well suited for many applications, are also generally expensive and have certain other drawbacks which the present invention is intended to eliminate.